This invention relates to underwater breathing apparatus of the type wherein breathing gas is circulated through a carbon dioxide absorbing cannister, and more particularly to an improved carbon dioxide absorbing cannister therefor.
There are numerous configurations of underwater breathing apparatus utilizing a carbon dioxide absorbing medium, such as a composition of sodium hydroxide or barium hydroxide, to remove carbon dioxide from the breathing gas so that it can be recirculated to the diver. In the case of mixed gas (usually helium and oxygen) diving to great depths, such recirculation is principally in the interest of economy and logistics relative to the acquisition and storage of large quantities of helium. A typical apparatus comprises a cannister or chamber in which a bed of particulate carbon dioxide absorbent material is confined, and through which the breathing gas is passed prior to recirculation to the diver. The motive power for accomplishing the recirculation is usually derived from make-up gas delivered at relatively high pressure to a nozzle that discharges into the throat of an ejector passage.
A major problem of long standing has been that of providing a carbon dioxide absorbing cannister, or scrubber, that will remain active and useful for the prolonged periods of time common to mixed gas dives. The mentioned compositions, as well as others, remove carbon dioxide through an exothermic reaction that requires the presence of water to proceed. Water in the absorbent material may be supplemented by the moisture in the exhaled breathing gas which often enters the cannister at, or close to, one hundred percent humidity, and substantially at ambient water temperature. Failure of the charge or bed of absorbent in the cannister to be effective through any period approaching its calculated effective life has generally been experienced and often provides the limiting factor to the length of a dive. Simply enlarging the absorbent bed has not helped to any useful degree, nor have various attempts at directing the gas flow to unused areas of the absorbent bed by baffling.
It is believed that such failures of the carbon dioxide absorbent cannisters are due to heating of the incoming air as a result of the exothermic reaction that occurs in a zone of the charge proximate to the inlet and extending for a portion of the length of the cannister. Although the incoming air is at or near one hundred percent relative humidity, as it passes into the mentioned zone it becomes heated, the relative humidity falls, and the air actually takes moisture from the remaining carbon dioxide absorbent charge. The remaining charge becomes drier and is unable to enter into the carbon dioxide absorbing reaction for lack of water.